1. Field of the Invention
The present invention relates to a control method for a microfilm image reader, in which a scanner reads an image of a frame recorded on a microfilm and supplies image data of the read image to a printer.
2 . Prior Art
A typical microfilm image reader includes a host computer, a scanner and a printer, in which the scanner retrieves a target frame from a microfilm based on a command sent from the host computer to the scanner, reads an image in the target frame and transfers image data of the read image to the host computer. The host computer then processes the image data in a predetermined image processing and the processed image data is either supplied to the printer or transferred to another computer.
The microfilm used herein generally contains black and white images which are read as binary density images by the scanner and supplied to the printer.
However, other images, which largely contain middle or half tone image (gray scale image) portions such as photographs, signs and stamps, are often recorded on the microfilm in addition to the black and white document images. When printing out such images containing middle tone image portions by a conventional printer, density data of the images must be binarized by a proper threshold value, followed by image processings for representing the middle tones, but it has been difficult to set such a threshold value.
Although a method may be used to set the threshold value for binarization processing, in which a density or gray level histogram of an image is plotted to determine the threshold value, the determination process has been complicated. Even if a proper threshold value is determined, false contouring lines like as a moire pattern often appear especially in middle-tone portions of the image when printed out. To prevent the occurrence of the false contouring, a processing such as a dither processing needs performing.
However, since a dither matrix needed for the dither processing must be changed depending on characteristics of the image to be processed, the processing has been too complicated. Further, when such a particular image processing is performed, delicate shades of the original image may not be represented, and it has been difficult to print out image data of the original image accurately.